1. Field of the Invention
The present invention generally relates to rotors of dynamoelectric machines, such as generators used in the production of electrical power. More particularly, this invention relates to a retaining ring for supporting field end windings of such a rotor, wherein the retaining ring has a composite construction that provides a high-strength, low-density support structure for the field end windings.
2. Description of the Related Art
Large turbine-driven generators used in the production of electrical power comprise a rotor that serves as a source of magnetic lines of flux produced by a wound coil carried on the rotor. The rotor rotates within a stator that comprises a number of conductors in which an alternating current is induced by the rotor as it rotates within the stator, generating a rotating magnetic field in a narrow air-gap between the stator and rotor.
A rotor 10 illustrative of the type used in turbine-driven generators is depicted in FIGS. 1 and 2. The rotor 10 is generally a large cylindrical body from which spindles 12 extend for rotatably supporting the rotor 10. The rotor 10 has a series of longitudinal (axially-extending) slots 14 machined radially into its outer circumference, which results in radially-extending teeth 15 being defined along the perimeter of the rotor 10. Field windings 17, comprising multiple insulated conductor strands, are installed in the slots 14 to extend the length of the rotor 10, longitudinally projecting from each end 26 of the rotor 10. The field windings 17 include end turns 18 (FIG. 2), each of which electrically connects the longitudinal portion of a winding in one slot 14 with the longitudinal portion of a winding in an adjacent slot 14. The field windings 17 do not fill the entire slot 14, which typically has a tapered region so that the slot 14 is narrower at the perimeter of the rotor 10. Wedges (not shown) are placed in the tapered region of each slot 14 to hold the windings 17 in place against centrifugal forces exerted when the rotor 10 rotates.
As the rotor 10 spins, the end turns 18 are also subjected to centrifugal forces that urge the end turns 18 radially outward. This radial movement of the end turns 18 is confined by retaining rings 16 that are attached to the ends of the rotor 10 to enclose the end turns 18, as shown in FIG. 2. As is widely practiced, retaining rings 16 of the type shown in FIG. 2 are attached to the ends of the rotor 10 by shrink fitting. In FIG. 2, the inboard end of the retaining ring 16 is shrink-fit around a shoulder 20 defined on the rotor 10, and a locking key 22 is provided between the ring 16 and rotor 10 to prevent axial movement of the ring 16. The retaining ring 16 is also supported at its outboard end with a centering ring 24, onto which the ring 16 is also preferably shrink-fitted.
Centrifugal forces generated as a result of the spinning rotor 10 cause the end turns 18 to press firmly against the inside surface of each retaining ring 16, applying a considerable force to the rings 16. Consequently, the retaining rings 16 are typically formed of a high-strength, nonmagnetic steel such as 1818 material. Sources of 1818 steel are limited and delivery cycles can be long, resulting in high costs. As rotor diameters and spin speeds increase, so do the centrifugal forces applied to the rings 16 by the end turns 18. However, because of the density of 1818 steel, as rotor spin speeds increase a significant part of the radial thickness of the ring 16 is required to resist the centrifugal forces generated as a result of its own weight. Furthermore, separation of the ring 16 and rotor 10 becomes a design challenge at higher rotor speeds.
In view of the above, lower-density composite retaining rings have been proposed, as well as other types of retaining systems. Examples include an epoxy-graphite retaining ring taught in commonly-assigned U.S. Pat. No. 5,068,564 to Frank. There is a demand for further improvements in the construction and implementation of low-density retaining rings.